1. Field of Invention
This invention pertains generally to the field of guided microwave spectroscopy and more particularly to measurement cells used to implement such spectroscopy techniques.
2. Description of Prior Art
Guided microwave spectroscopy (GMS) is a system that combines microwave spectral technology with a waveguide to provide precise compositional analysis of flowable materials, which can either be slurry or a gas. In particular, the GMS system permits the measurement of changes in the dielectric constant and the conductivity of the flowable material, thereby allowing the moisture content and other constituent properties to be inferred. An exemplary embodiment of GMS technology is disclosed in U.S. Pat. No. 5,455,516 entitled METER AND METHOD FOR IN SITU MEASUREMENT OF THE ELECTROMAGNETIC PROPERTIES OF VARIOUS PROCESS MATERIALS USING CUTOFF FREQUENCY CHARACTERISTIZATION AND ANALYSIS, issued to Jean et al. on Oct. 3, 1995. FIG. 2 depicts the frequency sensitive measurement cell 220 that is used for performing GMS measurements as disclosed in the Jean patent. Measurement cell 220 is connected in line with and forms a portion of a conduit carrying the flowable material being examined. The Jean device includes transition members 210 and 212 which are necessary to convert the circular cross section of the material processing conduit 208 into the rectangular cross section of the measurement cell 220. For highly viscous materials or materials such as a gas transported under relatively high pressure, the transition members introduce discontinuities in the mass flow rate of the material, affecting the accuracy of any measurement derived from the GMS system as well as the integrity of the production line which is processing the material.
An example of a highly viscous flowable material is corn masa, which is a dough used for baking tortillas. A tortilla is a baked grain product which originated in Mexico and is now widely consumed throughout the world. As the demand for tortillas has grown, the methods and the apparatus for automatically producing tortillas in high volumes have become well known. In the conventional automated system, dough is produced by cooking whole corn and grinding it while in a wet state or by combining instant corn masa flour with water in a commonly available mixer. This dough is generally referred to as “masa”. However, the term “masa” as used herein refers to this corn dough and other dough or slurry like materials having similar characteristics. The masa is often fed into an extruder which compresses the masa and outputs it in the form of a generally continuous stream to a pneumatic cutter. The pneumatic cutter chops the masa into generally cylindrical pieces, generally known as “logs”. The logs are usually carried on a conveyor to a masa hopper, which gravity feeds the masa to several successive pairs of generally opposed, counter rotating cylindrical rollers for compression into a sheet having the thickness required for tortilla production. This “sheeted” masa is then cut into the desired tortilla shape by a commonly available rotary cutter, which usually cuts circles of varying diameter. The cut masa is then baked and/or fried, cooled, and packaged by commonly available commercial food processing equipment.
Depicted in FIG. 1 is a masa handling system that is a part of a larger arrangement of apparatus intended for the commercial production of tortillas or other food having masa dough as an ingredient. The general arrangement of the masa handling system 10 includes a commonly available commercial mixer 12 that is located at the beginning of the production line. The mixer 12 has a pivoting door 14 which can rotate downward towards a masa extruder 16. The masa extruder 16 compresses the masa 18 and feeds a generally continuous masa stream 20 through a nozzle 22. Two vertically opposed and aligned endless belt separator conveyors 24 and 26 have moving surfaces 28 and 30, respectively, which face each other. The longitudinal ends 32 and 34 of separator conveyors 24 and 26 are mounted adjacent to the nozzle 22. One of the two separator conveyors 24 is “L” shaped and has a vertical section or portion 36 and a horizontal section or portion 38 which terminates above a masa hopper 40.
The vertical section 36 of the “L” shaped separator conveyor 24 extends longitudinally below the longitudinal end of the other separator conveyor, thereby providing a moving surface opposite from the nozzle 22. A deflector plate 41 is mounted on the end of the other separator conveyor 26. The separator conveyors 24 and 26 move the masa 18 to the masa hopper 40 which contains the sheeter assembly 120. The masa hopper 40 must be supplied with masa 18 periodically.
A selectively operable diverter gate 42, for periodically permitting replenishment of the masa hopper 40, is located adjacent to the end 44 of the horizontal section 38 of the “L” shaped separator conveyor 24. The diverter gate 42 is shown in its open position. When the diverter gate 42 is closed, its top surface forms a gravity slide that feeds material to a horizontal feed conveyor 46, which in turn feeds another masa hopper 48.
Each masa hopper 40 and 48 has a hollow inner gravity feeder portion 50 containing two counter rotating shafts 52 mounted above a pair of primary rollers 54. The primary rollers 54 are, in turn, mounted above a pair of counter rotating sheet rollers 56 and a common rotary cutter. A horizontal tortilla conveyor 58 is mounted below the rotary cutter and has tortillas 60 on its upper surface. The remainder of the system can include various combinations of commonly known and widely available commercial food processing apparatus such as an oven, a cooling rack, and a packaging system.
As can be seen in FIG. 1, the mass flow analysis of the masa would ideally be performed in the region of the nozzle 22, but such a measurement cannot introduce any significant impediment to the flow of the masa 18 without endangering the required constant and continuous supply of masa to the conveyors 26 and 24. The rectangular measurement cell and its associated cross sectional transitions as shown in FIG. 2 would be unsuitable for use in such a corn masa processing environment.
What is needed when processing masa, other highly viscous materials or a relatively high pressure material is the flat plate geometry of the Jean measurement cell residing within an existing circular conduit that would permit the use of the flat plate measurement cell while employing the GMS technique.
An example of a conduit using parallel plates within a circular conduit is disclosed in U.S. Pat. No. 3,500,182, entitled APPARATUS FOR AND A METHOD OF HIGH FREQUENCY ELECTROMAGNETIC MOISTURE MEASUREMENT IN HIGHLY VISCOUS PASTES AND SIMILAR MATERIALS, issued to Reed et al. on Mar. 10, 1970. The Reed device measures moisture by passing high frequency electromagnetic signals through the viscous material. The viscous material is constrained in a chamber having a pair of opposed boundary plates extending edge on in the direction of movement of the material to form a combining guide path for the signals. The signals are evaluated before and after traveling through the material to determine the moisture content of the material. However, the Reed device processes the data received quite differently than the GMS apparatus, the GMS system being adapted to analyze complex permittivity properties. Further, the Reed device utilizes a different frequency which precludes launching the electromagnetic energy into the matrix under test in the TE10 mode as required by the GMS system. U.S. Pat. No. 4,630,316, entitled TRANSITION BETWEEN RECTANGULAR AND RELATIVELY LARGE CIRCULAR WAVEGUIDE FOR A UHF BROADCAST ANTENNA, issued to Vaughan on Dec. 16, 1986 exposes the difficulty of propagating linearly polarized TE10 and circularly polarized TE11 electromagnetic energy in a single transmission path. The present invention addresses the foregoing problems.